1. Field of the Invention
The present invention relates to the field of television signal processing. More specifically, the present invention relates to line-specific decoding of a vertical blanking interval (VBI) scan line.
2. Background Information
Television transmission signals typically include a vertical blanking interval (VBI). The VBI refers to the time required by the electron gun in a conventional television set (or similar visual display) to reset itself to the top of the television screen from the bottom of the screen. Conventional television sets and many other visual displays generate their display by using an electron gun(s) to illuminate each line on the television screen. For instance, the National Television System Committee (NTSC) standard currently used in the United States illuminates every other scan line starting at the top of the screen, then resets itself to the top of the screen once it reaches the bottom. During this reset from the bottom of the screen to the top of the screen, no lines of the screen are being illuminated. This period of reset is referred to as the VBI.
Typically, a portion of the VBI is used by television broadcasters for control information necessary for displaying audio and video information. This control information includes, for example, vertical and horizontal synchronization signals. In addition, a portion of the VBI is typically reserved for broadcasting the text for closed captioning for the hearing impaired. However, unused time remains in the VBI which can be used by a broadcaster to broadcast any of a wide variety of data, typically referred to as teletext data, of the broadcaster""s choosing.
A plurality of scan lines occur during the VBI. VBI scan lines are typically broadcast in accordance with one of a number of industry standard formats. For example, teletext scan lines are often broadcast in accordance with the North American Basic Teletext Specification (NABTS)1. Since hardware systems processing VBI scan lines typically operate with their own internal system clocks, virtually all industry standard formats call for the inclusion of a leading run-in clock at the beginning of a VBI scan line, to facilitate synchronization and decoding of the VBI scan line. For example, NABTS calls for a modulated data bit rate of 5.7272 MHz, and the inclusion of 8 run-in clock cycles at the beginning of a teletext scan line, followed by phase-aligned modulated data bits to be decoded.
1See Joint EIA/CVCC Recommended Practice for Teletext North American Basic Teletext Specification (NABTS), EIA-516, May 1988. 
As such, a VBI decoder typically includes functions for recovering the starting point of the run-in clock. The VBI decoder adjusts the average starting point with the actual starting point of each run-in clock that is encountered. The decoder maintains the average starting point such that it can be used with each scan line. The run-in clock average starting point is one of many xe2x80x9coperational parametersxe2x80x9d that are utilized, adjusted, and maintained by the decoder in decoding a scan line. A typical prior art VBI decoder uses the same average starting point parameter for each scan line within the VBI. While the average starting point parameter may be adjusted after a predetermined period of time, only a single starting point parameter is used for each scan line.
A second operational parameter used in decoding the scan line is the average voltage level of the run-in clock. The run-in clock recovery portion of the VBI decoder may include a function for computing the average voltage level of the run-in clock. This average voltage level is then used as a threshold voltage during the decoding of the scan line data. For instance, if a voltage greater than the average voltage level is detected, it is considered a logical one. If a detected voltage is less than the average voltage level, it is considered a logical zero.
The average voltage level parameter may also be used in detecting the starting edge of the run-in clock for the following scan line. A typical prior art decoder uses the same average voltage level parameter from a first scan line for detecting the leading edge of the following scan line received. Although the average voltage level parameter may be adjusted with each scan line decoded, only a single average voltage level parameter is maintained.
Another set of operational parameters maintained by the scan line decoder are anti-ghosting filter parameters. Ghosting results from an echo(es) of the transmitted signal. Ghosting occurs when there is a reflection of a television signal or similar xe2x80x9cbouncingxe2x80x9d of the signal off of a particular object in or near the signal""s path. Ghosting presents a problem when transferring data over the VBI because the reflected or echoed signal(s) interferes with the data being transmitted.
Several anti-ghosting mechanisms have been developed to reduce the effects of ghosting in a television transmission signal. Two such mechanisms are referred to as Adaptive Baseband Equalization (ABBE) filtering and Lookup Table (LUT) filtering. Both of these anti-ghosting filtering mechanisms look at one or more surrounding samples for a particular sample of data and perform various filtering processes using these bits to reduce the ghosting effects on the signal being transferred. Each of these filtering methods includes the step of applying one or more filter parameters to the scan line. The filter parameters are adjusted over time to ensure that the filtering methods remain effective.
Similar to the run-in clock characteristics, the same adjusted filter parameters are applied to each scan line received within a single VBI. Thus, a prior art decoder treats every scan line received as if it has similar signal characteristics, thus utilizing the same operational parameters.
However, with VBI transmissions, each scan line may have different characteristics based upon its content, source and transmission. For instance, the NABTS defines 21 scan lines that are available within a given VBI. Certain scan lines, or groups of scan lines, may be dedicated to a particular broadcaster. For instance, scan lines 10-13 may be dedicated to a first broadcaster, and scan lines 14-15 may be dedicated to a second broadcaster. Because the first and second broadcasters may have different encoding methods, scan lines 10-13 may have different characteristics than scan lines 14-15. Moreover, scan lines 10-13 may be transmitted over different transmission channels than lines 14-15, creating different signal characteristics. For example, a local broadcaster may insert teletext data on lines 10-13, while passing teletext data sourced from a nationwide network on lines 14-15.
As such, it is desirable to provide for line-specific decoding of scan lines broadcast within a VBI. It is further desirable to provide for a decoder that adjusts and maintains a separate set of operational parameters for each scan line received within a given VBI.
A system is programmed with a line-specific VBI scan line decoder. The decoder receives a scan line from a capture driver, the scan line having associated with it a scan line number. Using this scan line number as an index, the decoder retrieves a set of operational parameters from a storage medium. These operational parameters correspond only to the scan line number of the scan line received from the capture driver. The line-specific decoder then decodes the scan line using the operational parameters. For one embodiment, the decoding step also includes run-in clock recovery and anti-ghosting filtering. Once the scan line has been decoded, the operational parameters are adjusted based upon the results of the scan-line decoding.